The present invention relates to a method for bidirectionally pumping an optical fibre amplifier and to a bidirectionally pumped optical amplifier. The present invention also relates to a method for stabilizing the optical emission of a laser.
Erbium doped fibre amplifiers useful for telecommunication systems are pumped by high-power semiconductor lasers. In particular, high-power 1480 nm or 980 nm quantum well diode lasers are generally used. These lasers typically have Fabry-Perot optical cavities with no longitudinal mode selectivity and may emit over a broad wavelength range. However, the lasers must emit within an absorption band of the rare-earth ions in order to pump the amplifier. In the case of the 980 nm pumped amplifiers, the absorption band of erbium may be less than 15 nm wide, whereas the gain spectrum of a 980 nm pump laser may be as wide as 60 nm. Pump lasers must then meet stringent wavelength requirements and be immune to effects that might change the laser spectrum.
In the following of the description, a parameter called xe2x80x9cfree running wavelengthxe2x80x9d of the pump laser will indicate the operative wavelength of the laser, that is, the wavelength value of the peak of the gain spectrum of the pump laser when it is driven by a predetermined current. As the skilled in the art readily appreciates, in dependance of the driving current (or in dependance of the output power) of the laser the free running wavelength shifts, starting from a value xcexth, corresponding to the threshold current Ith of the laser. xcexth is generally referred as xe2x80x9cthreshold wavelengthxe2x80x9d.
A known method for stabilizing the wavelength emission of a 980 nm pump laser is the use of a low reflectivity grating coupled to the output, anti-reflection coated, low-reflectivity facet of the laser. See, for example, a first article of Giles et al., Reflection-Induced Changes in the Optical Spectra of 980-nm QW Lasers, IEEE Photonics Technology Letters, Vol.6, No.8, August 1994. According to Giles et al.""s first article, the use uf the grating allows the reduction of the sensitivity of the pump laser to weak reflections that affect the laser emission spectrum.
In a second article (Giles et al., Simultaneous Wavelength-Stabilization of 980-nm Pump Lasers, IEEE Photonics Technology Letters, Vol.6, No.8, August 1994), the same authors disclose the simultaneous wavelength-locking and stabilization of three 980-nm pump lasers, connected to the input ports of a 4xc3x974 fibre star coupler, through reflection from a single narrow-band fibre grating connected to one output port. According to Giles et al.""s second article introduction, injection-locking is a means for stabilizing laser sources, but may not be practical for the compact, low-cost sources required to pump the erbium doped fibre amplifier.
Injection-locking is a well known technique used to achieve single-longitudinal-mode operation of a multi-longitudinal-mode semiconductor laser by suppressing the side modes with continuous wave single-longitudinal-mode master laser injection phase-matched with the output emission of the injected laser and of a wavelength comprised in a locking bandwidth that ranges from 100 MHz to some GHz around the output emission xcex. This technique is presently used in optical systems for precisely selecting wavelength emission of laser transmitters in a bandwidth of about 100 MHz; it has also been proposed for reducing frequency-chirped dynamic linewidth in directly modulated single-longitudinal-mode semiconductor laser transmitters (see for example C. Lin, J. K. Andersen, Frequency chirp reduction in a 2.2 Gbit/s directly modulated InGaAsP semiconductor laser by cw injection, Electronics Letters, Jan. 17, 1985, Vol.21 No.2).
FIG.1 shows a configuration of a known bidirectionally pumped optical fibre amplifier 100, comprising an amplifying doped optical fibre section 101, for example an erbium doped amplifying fibre, pump lasers 102, 103, WDM couplers 104, 105, optical isolators 106, 107 for light signals, input and output terminals 108, 109. A signal light is launched in the amplifier 100 through the input terminal 108, travels along the doped fibre section 101 to be amplified therein and exits through the output terminal 109. Suitable energy for amplification is provided by pump lasers 102, 103, which couple pump light to the doped fibre 101 through WDM couplers 104, 105. In particular, pump light from laser 102 is launched co-directionally in the doped fibre 101, that is in the same direction of the signal light, whereas pump light from laser 103 is launched counter-directionally, that is, in the opposite direction with respect to the signal light. For an erbium doped fibre amplifier, pump lasers 102, 103 may emit light whose wavelength is comprised in a pumping band centered around 980 nm or 1480 nm.
Herein and in the following of the description, the expressions xe2x80x9cco-directionallyxe2x80x9d, xe2x80x9ccounter-directionallyxe2x80x9d, xe2x80x9cco-propagatingxe2x80x9d, xe2x80x9ccounter-propagatingxe2x80x9d will be always referred to the propagation direction of the signal light.
The configuration shown in FIG. 1 has a problem in that the residual pump light from each pump laser, not fully absorbed by the amplifying fibre, is injected into the opposite pump laser, which can result in optical instabilities and fluctuations in amplification of the optical signal.
It is known that such instability can be avoided by placing an isolator on the optical path of each of the pumps.
In patent U.S. Pat. No. 5,640,268 to Alcatel N.V. a solution is addressed to this problem. According to the ""268 patent, each pump injection fibre includes a photorefractive pump filter constituting part of the resonant cavity of the associated pump laser, the two pump filters being mutually different to give rise to an offset between the two pumping bands. The two pumping bands are preferably offset by several nanometers. The pump filters are photorefractive gratings having a determined pitch and thus a determined central wavelength for reflection: the use of such a grating makes it possible simultaneously to reduce the width of the pumping band and to position said band more accurately within the spectrum.
Applicant has experimentally verified that in a configuration according to the ""268 patent, if the wavelength emitted by the pump lasers is within the 980 nm pumping band the offset between the two pumping bands should be greater than 15 nm, in order to avoid instabilities due to residual pump injection. Since for an erbium doped fibre amplifier, the pumping band centered around 980 nm is only 10-15 nm wide, an offset between the two photorefractive filters of 15 nm or more would lead one of the pump wavelenghts to be nearly out of the pumping band of erbium, considerably reducing the bidirectional pumping efficiency.
Applicant has found that a pump laser, even without a stabilizing grating, can have a stable optical emission if it is injected by an external radiation having a wavelength close to the free running wavelength of the injected laser and having a sufficiently high power. A locking of the optical emission of the injected laser around the wavelength of the injection takes place. The useful xe2x80x9clocking bandwidthxe2x80x9d, that is, the useful difference between the injected wavelength and the free running wavelength of the injected laser may range up to several nanometers. Stability of the optical emission means that at least 80% of the power emitted by the injected laser is comprised in a wavelength range of about 2 nm around the wavelength of the injection.
Applicant has also found that in a bidirectionally pumped optical amplifier a pump residual due to a pump radiation not absorbed in an active fibre, said pump radiation coming from a first pump laser used with a stabilizing grating, may have a sufficient power for stably locking the optical emission of a second pump laser, used without a stabilizing grating. This can lead to substantial elimination of the optical instability of the injected pump laser in the operative condition of the bidirectionally pumped amplifier. In such configuration, the first laser acts as a master and the second laser acts as a slave, in a master-slave configuration, achieving bidirectional pumping of the amplifying fibre with a pump light having the same wavelength traveling co and counter-directionally.
In a first aspect, the invention relates to a method for pumping an optical amplifier comprising an active optical fibre, a first pump laser and a second pump laser, the method comprising:
coupling a first pump radiation at a predetermined wavelength emitted by the first pump laser in the active fibre in a first direction,
coupling a second pump radiation emitted by the second pump laser in the active fibre in a second direction, opposite to the first direction,
characterized by further comprising
coupling a first pump residual in the first direction from the active fibre into the second pump laser, so as to lock the emission wavelength of the second pump laser around said predetermined wavelength.
Preferably, the difference between the free running wavelength of the second pump laser and said predetermined wavelength is lower than 18 nm, more preferably lower than 8 nm, even more preferably lower than 5 nm.
In preferred embodiments, said predetermined wavelength and said free running wavelength are comprised between 968 nm and 986 nm.
Preferably, a first power ratio between an output power of the second pump laser and a power of the first pump residual is lower than 15 dB, more preferably lower than 10 dB, even more preferably lower than 8 dB.
In an embodiment, the locked emission wavelength of the second pump laser is comprised in an emission bandwidth of at least 0.5 nm. In a further embodiment, the locked emission wavelength of the second pump laser is comprised in an emission bandwidth of about 2 nm.
In a second aspect, the invention relates to a bidirectionally pumped optical amplifier comprising:
an active fibre having two ends,
a first WDM coupler and a second WDM coupler coupled to said ends,
a first pump branch coupled to the first WDM coupler comprising a first laser and a selective reflector, for introducing pump radiation having a predetermined wavelength in the active fibre in a first direction,
a second pump branch coupled to the second WDM coupler comprising a second laser, for introducing pump radiation in the active fibre in a second direction, opposite to the first direction,
characterized in that
the amplifier is adapted for coupling a pump residual from the active fibre into said second laser, the pump residual being selected so that the emission wavelength of the second laser is locked around said predetermined wavelength.
Advantageously, the first pump branch further comprises an optical isolator for the pump radiation.
Preferably, the difference between said predetermined wavelength and the free running wavelength of the second laser is lower than 18 nm, more preferably lower than 8 nm, even more preferably lower than 5 nm.
In preferred embodiments, said predetermined wavelength and said free running wavelength are comprised between 968 nm and 986 nm.
Preferably, a power ratio between an output power of said second laser and a power of said pump residual is lower than 15 dB, more preferably lower than 10 dB, even more preferably lower than 8 dB.
In an embodiment, the locked emission wavelength of the second laser is comprised in an emission bandwidth of at least 0.5 nm. In a further embodiment, the locked emission wavelength is comprised in an emission bandwidth of about 2 nm.
Preferably, a length of said active fibre is lower than 15 m. Preferably, an output power of at least one of the first and second-pump lasers is higher than 15 mW, more preferably higher than 50 mW, even more preferably higher than 100 mW.
Typically, the bidirectionally pumped optical amplifier according to the second aspect of the invention further comprises a feedback system for at least regulating the output power of the first and second pump lasers.
Preferably, the selective reflector is a grating, more preferably a fibre grating.
Typically, the first and second lasers are semiconductor lasers. In preferred embodiments, the semiconductor lasers are AlGaAs-InGaAs lasers.
In a preferred embodiment, an optical amplifier comprises a pre-amplifying section, including at least one active fibre and at least one pump laser, and a booster section, including a bidirectionally pumped optical amplifier according to the second aspect of the invention.
Advantageously, an optical amplifier comprising a bidirectionally pumped optical amplifier according to the second aspect of the invention may be included along the optical transmission path of an optical transmission system comprising at least one transmitter and at least one receiver, coupled to said optical transmission path. Preferably, the optical transmission system is a WDM system.
In a third aspect, the invention relates to a method for stabilizing the optical emission of a laser having a predetermined output power and a predetermined free running wavelength, the method comprising:
injecting into said laser a radiation having a wavelength comprised in a predetermined locking band around said free running wavelength, said injecting radiation having a sufficient power for thereby locking the optical emission of the laser around the wavelength of said injecting radiation, characterized in that the width of said locking band is at least 0.5 nm.
Preferably, the width of the locking band is lower than 15 nm.
Preferably, a power ratio between the output power of the laser and the power of the injected radiation is lower than 15 dB, more preferably lower than 10 dB, even more preferably lower than 8 dB.
In an embodiment, the bandwidth of the locked optical emission is at least 0.5 nm. In a further embodiment, the bandwidth of the locked optical emission is about 2 nm.
Typically, the said locked optical emission is multi-longitudinal mode.
In preferred embodiments, said free running wavelength is comprised between 968 and 986 nm.